1

Full Scale Fracture Toughness Behavior of

Zr-2.5Nb Pressure Tubes with High Hydrogen

Concentrations and Different Hydride

Morphologies

May 20, 2019

Jun Cui and Gordon K. Shek,

Kinectrics Inc, Toronto, Canada

19th International Symposium on Zirconium in

the Nuclear Industry

2

• Introduction

•Experimental Program

•Results and Discussion

•Conclusions

•Acknowledgements

OUTLINE

3

• Hydrogen Embrittlement in CANDU Pressure Tubes

❑ Cold-worked Zr-2.5Nb pressure tubes are used in CANDU reactors.

❑ Hydrogen

❑ Hydrides

INTRODUCTION

Circumferential

(or transverse)

Radial

Circumferential hydrides

formed without applied stress

Radial hydrides

formed at 350 MPa

OD

Hoop

Stress

Hoop

Stress

4

• Hydrogen Embrittlement in CANDU Pressure Tubes

❑ The detrimental effects of reoriented hydrides for lowing ductility and

fracture toughness of Zirconium alloys are known for years.

❑ Recently, there is a renewed interest on this issue because:

➢As the pressure tubes age and approach their design end-of-life, the hydrogen

concentration in the pressure tubes becomes significantly higher than before.

➢For in-service evaluation for fracture initiation and leak-before-break, there is a

need to determine the fracture toughness properties of the pressure tubes with

high hydrogen concentrations and different hydride morphologies.

➢Such information is essential to support continued reactor safe operation and

reactor life extension.

INTRODUCTION

5

• Hydrogen Embrittlement in CANDU Pressure Tubes

❑ An extensive experimental program has been initiated in the CANDU

industry to provide information to support development of fracture

toughness models. The test parameters include:

➢Irradiated versus unirradiated materials

➢Small CCT specimen versus full-scale burst test specimen

➢High hydrogen concentration and different hydride morphologies

➢Hydride reorientation cycles

➢Test temperature

INTRODUCTION

6

• Scope of This Paper

❑ This paper presents results obtained from rising pressure burst testsperformed on sixteen full-scale burst test specimens over a range oftemperatures to characterize the fracture toughness properties ofhydrided, unirradiated Zr-2.5Nb pressure tube material in the lowershelf, transition and at the onset of the upper shelf fracture regimes.

❑ The test variables include:

➢Material variability

➢Hydrogen concentration and hydride morphology

➢Test temperature

INTRODUCTION

7

• Material

❑ Two unirradiated, cold-worked Zr-2.5Nb pressure tubes Y041 andC022 were selected for testing from a total of 10 candidate tubes.

EXPERIMENTAL PROGRAM

8

• Specimen

EXPERIMENTAL PROGRAM

Illustration of a burst test specimen prior to fatigue pre-cracking

Axial through-wall notch

9

• Hydriding

❑ For each tube, burst test specimens with four different hydrogenconcentrations were prepared for testing: 6 ppm (as-fabricatedcondition), 60 ppm, 100 ppm and 130 ppm.

❑ For specimens that require hydriding, an electrolytic hydridingtechnique was used to deposit a hydride layer on both inner andouter surfaces of each specimen.

EXPERIMENTAL PROGRAM

10

• Hydride Reorientation

EXPERIMENTAL PROGRAM

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Time (h)

Te

mp

era

ture

(°C

)

50

60

70

80

90

100

110

120

130

140

150

160

170

Ap

plie

d H

oo

p S

tres

s (

MP

a)

Temperature Applied Hoop Stress

302°C/20h

Heatup: 1˚C/min; Cooldown: 0.7˚C/min.

60°C

300°C/1h

Illustration of hydride reorientation cycle for 60 ppm specimens

11

• Fatigue Pre-Cracking

❑ Prior to burst test, the EDM notch was sharpened by pressurecycling in de-ionized (DI) water to form ~5 mm crack at both ends ofthe notch using a decreasing K procedure with a final K of ~ 15MPa√m.

• Burst Test

❑ Argon gas was used to perform rising pressure burst test.

❑ For testing at elevated temperatures, an internal cartridge heater wasused to heat the specimen.

❑ The specimen was pressurized to failure while the crack extension was measured using the potential drop technique.

➢This is referred to as “Rising Pressure Burst Test”.

EXPERIMENTAL PROGRAM

12

• Characterization of Burst Test Fracture Toughness

❑ Two approaches are generally used to characterize fracture toughness of a burst test specimen.

❑ 1st approach uses Kc, the plane stress critical stress intensity factor at the onset of flaw instability.

➢This is calculated using the burst pressure and the initial crack length.

❑ 2nd approach uses J-R curve, the J-integral versus crack extension curve.

EXPERIMENTAL PROGRAM

13

• Post-Test Examination

❑ Fracture surface was examined to measure crack length and examine the influence of hydrides on crack growth behavior.

❑ Metallographic samples in the vicinity of the fracture surface were prepared to examine the hydride morphology in the radial-transverse plane.

❑ Hydride morphology was characterized by a parameter referred to as “Hydride Continuity Coefficient”, or HCC, see illustration on next slide.

❑ HCC varies between 0 and 1. HCC provides a measure of the extent to which the hydrides are reoriented towards the radial direction of the pressure tube wall, with a higher HCC corresponding to more reoriented hydrides.

EXPERIMENTAL PROGRAM

14

• HCC Characterization

EXPERIMENTAL PROGRAM

Radial

Transverse

15

• Test Matrix

EXPERIMENTAL PROGRAM

16

• Stage 1 Results: Hydride Morphology

RESULTS AND DISCUSSION

HCC=0.66 HCC=0.67 HCC=0.19 HCC=0.15

Radial

Transverse

Y041, 100ppm Y041, 130ppm C022, 100ppm C022, 130ppm

17

• Stage 1 Results: Crack Growth Behavior and Kc

RESULTS AND DISCUSSION

Comparison of J-R curves from Stage 1 burst test specimens

18

• Stage 1 Results: Summary

❑ Fracture Toughness Ranking:

➢S1-1 (Y041, 100 ppm) is the lowest toughness specimen.

➢ This is the Stage 2 material condition.

➢S1-4 (C022, 130 ppm) is the highest toughness specimen.

➢ This is the Stage 3 material condition.

➢S1-3 (Y041, 130 ppm) is the second lowest toughness specimen.

➢ This is the Stage 4 material condition.

RESULTS AND DISCUSSION

19

• Stage 1 Results: Fracture Surface

RESULTS AND DISCUSSION

20

• Stage 1 Results: Fracture Surface

RESULTS AND DISCUSSION

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• Stage 2 Results: Crack Growth Behavior

RESULTS AND DISCUSSION

Comparison of J-R curves from Stage 2 burst test specimens (Y041, 100 ppm)

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0

J-I

nte

gra

l (k

J/m

2)

Average Crack Extension (mm)

S1-1 (HCC: 0.66), 100 C

S2-2 (HCC: 0.45), RT

S2-3 (HCC: 0.47), 150 C

S2-1 (HCC: 0.54), 200 C

22

• Stage 2 Results: Fracture Surface

RESULTS AND DISCUSSION

23

• Stage 2 Results: Fracture Toughness Transition Behavior

RESULTS AND DISCUSSION

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• Stage 3 Results: Crack Growth Behavior

RESULTS AND DISCUSSION

Comparison of J-R curves from Stage 3 burst test specimens (C022, 130 ppm)

0.0

100.0

200.0

300.0

400.0

500.0

600.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

J-I

nte

gra

l (k

J/m

2)

Average Crack Extension (mm)

S1-4 (HCC: 0.15), 100 C

S3-1 (HCC: 0.27), RT

S3-2 (HCC: 0.28), 150 C

S3-3 (HCC: 0.30), 100 C

25

• Stage 3 Results: Fracture Toughness Transition Behavior

RESULTS AND DISCUSSION

26

• Stage 4 Results: Crack Growth Behavior

RESULTS AND DISCUSSION

Comparison of J-R curves from Stage 4 burst test specimens (Y041, 130 ppm)

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

J-I

nte

gra

l (k

J/m

2)

Average Crack Extension (mm)

S1-3 (HCC: 0.67), 100 C

S4-2 (HCC: 0.36), RT

S4-1 (HCC: 0.50), 150 C

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• Stage 4 Results: Fracture Toughness Transition Behavior

RESULTS AND DISCUSSION

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• Stage 5 and Stage 6 Results: Crack Growth Behavior

RESULTS AND DISCUSSION

Comparison of J-R curves from Stage 5 (Y041, 6 ppm [H], 4.1 ppm [Cl]) and

Stage 6 (C022, 6 ppm [H], 1.4 ppm [Cl]) burst test specimens

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• Stage 5 and Stage 6 Results: Fracture Surface

RESULTS AND DISCUSSION

SEM micrograph showing fracture

surface of S5-1, Y041, 4.1 ppm [Cl]

SEM micrograph showing fracture

surface of S6-1, C022, 1.4 ppm [Cl]

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• Stage 7 (Y041, 60 ppm [H], 4.1 ppm [Cl]) and Stage 8 (C022, 60 ppm [H], 1.4 ppm [Cl]) Results: Hydride Morphology

RESULTS AND DISCUSSION

HCC=0.19 HCC=0.15

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• Stage 7 and Stage 8 Results: Crack Growth Behavior

RESULTS AND DISCUSSION

Comparison of J-R curves from Stage 7 (Y041, 60 ppm [H], 4.1 ppm [Cl]) and

Stage 8 (C022, 60 ppm, 1.4 ppm [Cl]) burst test specimens

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• Effect of Material Variability

❑ Y041 vs C022 with as-fabricated [H].

➢S5-1 vs S6-1

❑ Y041 vs C022 with 60 ppm [H].

➢S7-1 vs S8-1

❑ Y041 vs C022 with 100 ppm [H].

➢This refers to burst tests on S1-1 (Y041) and S1-2 (C022). Both tests were performed at 100˚C. Under nominally identical hydride reorientation cycles, there was significantly more hydride reorientation in S1-1 (HCC: 0.66) than that in S1-2 (HCC: 0.19). The Kc of S1-1 was lower than that of S1-2.

RESULTS AND DISCUSSION

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• Effect of Material Variability

❑ Y041 vs C022 with 130 ppm [H].

RESULTS AND DISCUSSION

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• Effect of Hydrogen Concentration

RESULTS AND DISCUSSION

S6-1 (C022, 6 ppm, HCC: 0)

S8-1 (C022, 60 ppm, HCC: 0.16)

S3-1 (C022, 130 ppm, HCC: 0.27)

S5-1 (Y041, 6 ppm, HCC: 0)

S7-1 (Y041, 60 ppm, HCC: 0.19)

S2-2 (Y041, 100 ppm, HCC: 0.45)

S4-2 (Y041, 130 ppm, HCC: 0.37)

20

40

60

80

100

120

140

160

180

200

0 25 50 75 100 125 150

Kc

(MP

a√m

)

Hydrogen Concentration (ppm)

Y041: 4.1 ppm [CL]C022: 1.4 ppm [Cl]

35

• Effect of Hydride Morphology

RESULTS AND DISCUSSION

36

• Material Variability

❑ At room temperature, Tube Y041 had lower fracture toughness (Kc) than Tube C022 in as-received hydrogen condition, likely due to the difference in [Cl] and number of Zr-Cl-C fissures between these two tubes. In the presence of higher [H] and reoriented hydrides, [Cl] likely played a less prominent role affecting fracture toughness.

❑ Under nominally identical hydride reorientation conditions, there were similar amount of reoriented hydrides in 60 ppm hydrided specimens from Tubes Y041 and C022; there was a significant difference in hydride reorientation in 100 & 130 ppm specimens between these two tubes. This suggested the dependence of hydride reorientation on tube-to-tube variability with hydrogen concentration likely a factor.

❑ Overall, 100 & 130 ppm hydrided specimens from Tube Y041 had lower fracture toughness and exhibited a sharper transition behavior versus temperature and a higher transition temperature to upper-shelf fracture toughness than those from C022 containing a similar level of hydrogen concentration..

CONCLUSIONS

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• Hydrogen Concentration

❑ The results show an overall trend of decreasing Kc with increasing [H] from 6 ppm to 130 ppm. There was a significant reduction in Kc between 6 ppm and 60 ppm, a smaller reduction in Kc between 60 ppm and 100/130 ppm.

• Hydride Morphology and Transition Behavior

❑ Fracture toughness of the hydrided specimens exhibited a hydride morphology dependent transition behavior with test temperature.

❑ Specimens with relatively low HCC had a gradual transition behavior.

❑ Specimens with relatively high HCC values had a sharp transition behavior with a relatively high transition temperature.

CONCLUSIONS

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• Funding of this work was provided by CANDU Owners Group (COG) Inc., Canada.

• The authors would like to thank a number of folks who provided help and technical assistance at different stages of this project:

❑ H. Seahra (deceased), M. Mills, J. Schaefer, P. Vesely, N. Dignam.

❑ S. St. Lawrence, D. A. Scarth, A.C. Wallace, D. Graham and B. Mills.

ACKNOWLEDGEMENTS

39

BACKUP SLIDES

RESULTS AND DISCUSSION

40

• Specimen

EXPERIMENTAL PROGRAM

Illustration of a burst test specimen prior to fatigue pre-cracking

41

• Stage 1 Results: Burst Test Specimen Crack Appearance

RESULTS AND DISCUSSION